Well logs are commonly used in the field of oil and gas production and exploration to determine the nature and attributes of the geological formations surrounding wellbores. The logging of a well may, of course, be done in the process of drilling the well, to correlate the well to seismic surveys and to specifically analyze the formations into which drilling has taken place. Well logs are also used in the infill drilling stage of field development, and also in reservoir maintenance and field management at producing locations.
Various types of conventional well logs, using different physical mechanisms, are commonly used. For example, well logs may be taken by gamma ray measurements of the natural radioactivity of formations surrounding the borehole. Another type of log analyzes formation density by emitting gamma radiation into the formation and by measuring the back-scatter of gamma radiation from the formation. Conventional sonic logs measure sonic energy that has been reflected by the surrounding formation at multiple locations along the logging tool from the sonic source. Another type of conventional well log measures the electrical conductivity of the surrounding formation to identify whether fluids present in the formation are water or oil. These conventional well log measurements are generally used, individually, to provide indications of the properties of the sub-surface formations surrounding the location in the wellbore at which the well log measurements were taken.
These conventional well logging techniques are generally useful in analyzing the structure and composition of formations into which the drilling operation has progressed. Certain attributes of the formation are especially important in determining whether or not oil is present therein. The porosity of the formation is a measure of the fluid capacity of the formation, as it indicates the extent to which the volume of the formation is occupied by rock, or rather the extent of the volume of the formation that may instead be occupied by oil. Since a porous formation may be occupied by either water (generally saline water) or oil, water saturation of the reservoir fluid is another important parameter in determining whether viable oil production is available.
The parameters of porosity and water saturation may not be determined solely from well logs according to conventional analysis techniques. As described in my copending application Ser. No. 07/989,863, filed Dec. 14, 1992, and incorporated herein by this reference, conventional porosity determinations required extrinsic information regarding the lithology of the formation. As such, core sampling and material analysis is generally performed, which is necessarily an expensive undertaking, especially considering that drilling must stop and the drill string retrieved from the borehole in order to perform such sampling. Furthermore, if casing has already been installed in the borehole, core sampling is rendered impossible. While the lithology of the formations may also be estimated from analysis of drilling cuttings as they arrive at the surface during the drilling of the well, these estimates are generally poor considering that cuttings generated at the bit mix with material from the wellbore sidewalls at shallower depths, and may even be blocked from arriving at the surface.
Given that porosity cannot be directly determined from well log analysis, it is therefore apparent that conventional logging techniques are also unable to estimate the water saturation of the formation in a manner that does not also require extrinsic information regarding the lithology of the formation. For example, as noted above, conventional well log measurements have been used, individually and in combination with extrinsic information, to indicate certain properties of the surrounding formation. An example of such a conventional technique has been used to determine water saturation from the response of a pulsed neutron capture (PNC) log measurement. According to one such conventional approach, the PNC response .SIGMA..sub.a may be represented as: EQU .SIGMA..sub.a =.phi.S.sub.w .SIGMA..sub.aw +.phi.(1-S.sub.w).SIGMA..sub.ao +(1-.phi.).SIGMA..sub.am
where .SIGMA..sub.ao, .SIGMA..sub.aw, .SIGMA..sub.am are the log response components due to oil, water, and matrix, respectively, which respectively correlate to the absorption cross-section of oil, water, and matrix. In this equation, .PHI. is the porosity of the formation. According to this conventional method, the .SIGMA..sub.ao, .SIGMA..sub.aw, .SIGMA..sub.am, and .PHI. components are set to assumed values (either based on extrinsic information regarding the formation, or upon assumptions or estimates). Upon obtaining the PNC response .SIGMA..sub.a from the well log measurement, therefore, this conventional method was able to solve for the factor S.sub.w, which is the water saturation of the formation.
Of course, as is evident from the foregoing, either extrinsic information is required to provide the porosity and matrix absorption cross-sections, or the human analyst must venture a guess as to the value of these factors, in order to derive a water saturation value. The cost required to obtain such direct measurements, or the inaccuracy of guessing the value of these factors, renders such conventional water saturation analysis either impracticable or of minimal value.
It is therefore an object of the present invention to provide a method and system for determining the porosity and water saturation of formations surrounding the wellbore directly from well log data.
It is a further object of the present invention to provide such a method and system which does not depend upon extrinsic information regarding the lithology of the formations in order to determine the water saturation content of a formation.
It is a further object of the present invention to provide such a method and system which may be used in the logging of cased wellbores.
It is a further object of the present invention to provide such a method and system which can determine the salinity of the water saturation in the formation.
It is a further object of the present invention to provide such a method and system which uses certain conventional well log measurements in combination with one another to provide a more direct indication of the presence of hydrocarbons in the formations being logged.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the claims.